This invention relates to tuned radio frequency amplifiers and specifically for such amplifiers for use in FM radios driven from a low voltage DC power supply, especially such as are designed to be used in motor vehicles. This invention is particularly directed to such tuned radio frequency amplifiers including tank circuits containing back-to-back varactor diodes as the capacitance element thereof.
A tuned tank circuit for a radio frequency amplifier comprises a back-to-back varactor diode pair with its anodes connected across an inductor and its common cathode connected to a source of tuning voltage. Input or output for the circuit is generally made across the anodes; and variation of the tuning voltage causes a change in the capacitance of the back-to-back varactor diode pair with consequent change in frequency of the tank circuit. Since it is generally desired to tune a specified range of frequencies, such as the FM radio band of 88 to 108 Megahertz, a certain minimum range of regulated tuning voltage is required. In applications such as vehicle mounted radios, the limited voltage range of the standard vehicle electrical generating and storage system creates difficulties in obtaining a full tuning voltage range for the FM band.
For example, a standard vehicle electrical generating system provides a potential of 10 to 16 volts which can be regulated for tuning voltage purposes to a constant voltage having a maximum value of approximately 8 volts. With the standard varactor diode tank circuit connections, the range of tuning voltage must extend from slightly below 8 volts to a voltage below 2 volts. However, operation at less than 2 volts produces a number of difficulties. The varactor diode pairs become more nonlinear, with resultant increases in temperature instability and difficulty of tracking parallel tuned circuits; the varactor diode pairs also become more susceptible to overload from forward bias with resultant distortion. If, however, it is desired to keep the minimum tuning voltage above 2 volts, a more expensive regulated power supply is required to boost the available regulated voltage. Both of these approaches have been attempted in the prior art.